1. Field of the Invention
The invention relates to the field of electrical sockets and connectors and more particularly to a DIP socket for mounting integrated circuit devices onto printed circuit boards.
2. Description of the Prior Art
Dual-in-line-package (DIP) sockets are well-known in the prior art. Integrated circuits are commonly encapsulted in rectangular housings having a number of flat leads disposed in two parallel rows extending from opposite sides of the package, hence the name dual-in-line-package. Generally, it is not desirable to mount an integrated circuit package directly onto a printed circuit board since the heat of soldering the package leads to the board may harm or destroy some of the delicate microelectronic circuits within the package. Additionally, it is frequently useful to be able to readily remove and replace a particular integrated circuit package which may be defective without having to desolder its connections to the printed circuit board. To this end DIP sockets were developed in order to provide a relatively safe and reliable interconnection between integrated circuit packages and a printed circuit board. These sockets generally include a plastic or ceramic housing having rows of receiving holes containing socket terminals which are in alignment with the leads on the integrated circuit package which is to be connected. The DIP socket is designed to have its terminals fitted into via holes formed in a printed circuit board, soldered in place, and then to have an integrated circuit package inserted into the socket terminals of the DIP socket to make an electrical interconnection with the printed circuit board. In this manner, the DIP socket may be soldered by conventional techniques, such as wave-soldering, without in any way harming the electronic devices within the integrated circuit package. Additionally, the integrated circuit package is readily removable which aids in assembly and repairs.
However, two major problems have existed in prior art DIP socket technology. The first is in ensuring a positive, reliable and corrosion free interconnection between the leads of an integrated circuit package and the contacts of a DIP socket. Prior art devices have attempted to overcome this problem by providing cantilevered leaf spring contacts which securably grip an inserted lead. Workers in the prior art have also recognized that the tighter the seal between the inserted lead and its associated leaf spring contacts, the less likely oxidation and corrosion is to occur. However, the higher contact pressure associated with these contacts means that the force needed to insert a lead into the contacts is correspondingly higher. High insertion forces make it difficult, if not impossible to use automatic insertion machinery, since the machinery will tend to bend the contacts or leads of the components if there is any misalignment between the integrated circuit package, the DIP socket, or the printed circuit board. Ideally, the contacts themselves should also be readily insertable into the DIP socket housing and have positive locking and retention means to prevent contact loss during the various manufacturing steps associated with these socket devices. Additionally, higher insertion forces and contact pressure tend to scrape or "wipe" the metallization of both the contacts and the inserted leads. A certain amount of wipe is desirable since it helps to remove any surface oxidation or dirt on the contacts and leads. However, if it is desired to insert and remove the leads of the integrated circuit package from prior art DIP sockets more than two or three times, the high contact pressures and insertion forces of these prior art sockets often will completely strip or erode the protective metallization off of the contacts and leads, resulting in a poor, incomplete, or noisy connection and subsequent degradation of circuit performance.
Workers in the prior art have recognized that insertion forces may be substantially reduced by preloading or prestressing the contacts of the DIP socket while maintaining a high normal force or pressure against an inserted lead. Examples of such prior art preloading methods include those where small ears or spurs are formed on the ends of the contacting spring arms and are designed to fit behind mating slots in an interior cavity of the DIP socket housing. When the contact is inserted into the housing the ears on the ends of the contact arms will latch into the mating slots, slightly biasing the spring arms apart. Another method of preloading is shown in U.S. Pat. No. 3,865,462 which uses vertically disposed ramps within the socket cavity to bias apart adjacently disposed leaf spring members. A third method of preloading uses a tapered pin inserted between the contacts to bias the contacts apart so as to cause them to latch behind ledges provided within the socket housing. A fourth method of preloading is shown in British Pat. No. 1,443,288 in which the spring arms of a contact are urged apart by a pair of angled rampways, formed in an upper portion of a two piece contact receiving header, when the header portions are joined together. However, all the above prior art methods require a fairly high insertion force for inserting the contacts into the housing, require hard to machine spurs or latches and corresponding ledges to retain the contacts in their preloaded position, require a separate preloading or biasing tool to be inserted into the socket to effectuate contact preloading, or require a multipiece header with its concomitant increase in manufacturing and assembly costs. Further, prior art preloading methods tend to use portions of the contacting area itself, biased against ledges or ramps within the socket housing, to effectuate preloading. However, such techniques tend to be unreliable since the contacting area itself must make frictional contact with the ramp or ledge provided within the housing which may cause the surface of the contacting area to be damaged or eroded during a preloading operation.
Another problem with prior art preloaded contacts is that although preloading generally reduces the insertion force for a terminal to be inserted into a contact, the very same preloading techniques generally do not allow for controlling the normal forces on the inserted terminals after insertion. In most prior art techniques, after a terminal is inserted into a contact, the preloaded contact arms will lift off whatever preloading means are used into a non-contacting position with respect to the preloading means. Hence, the only force available as a normal force to an inserted terminal lead will be that provided by the spring constant of the contacting arms of the contact. Generally, this force is much lower than desirable for good contact-to-terminal electrical connectivity and does not provide adequate protection of the contacting area against oxidation and corrosion.
A second major problem encountered in prior art DIP socket technology is that of solder wicking. When a DIP socket is soldered to a printed circuit board, usually by means of wave soldering, there is a tendency for the solder and flux to "wick" up the DIP socket terminals, usually by capillary action. If flux enters the area of the leaf spring contacts within the DIP socket housing it may inhibit the proper action of the spring contacts or cause a faulty connection. If solder is wicked up into the contacting area it can "freeze" together the contacts and prevent electrical device leads from entering. Prior art DIP sockets have dealt with the solder wicking problem in a variety of ways. A common method is to use a long contact tail, to be inserted into the printed circuit board, in conjunction with high standoffs or spacers provided on the underside of the DIP socket housing to raise the DIP socket a distance away from the printed circuit board. However, this method merely slows down capillary action along the terminal, but does not prevent it altogether. Other prior art methods are shown in U.S. Pat. Nos. 3,717,841 and 3,989,331 which use DIP socket terminals having solder-resist compounds placed along a portion of their terminals to prevent capillary solder flow. Such methods suffer from the obvious defect that a special localized solder-resistant coating must be applied to a portion of each terminal. In addition, these solder resist coatings have a tendency to become brittle after a period of time and will tend to break off or flake during use, thus reducing their antiwicking properties and possibly interrupting circuit interconnections between IC leads and socket contacts. Another method, shown in U.S. Pat. No. 4,010,992, uses a friction fitting seal in the lower portion of the DIP socket body through which the contact terminal is inserted. The friction seal acts to prevent solder and flux from flowing up the terminal and into the interior of the contact cavity. However, such a method requires a more complex DIP socket housing to accommodate the plurality of friction seals needed for each DIP socket. Another technique, shown in U.S. Pat. No. 3,525,972, uses a pair of facing tail members having a bulbous section formed therebetween to prevent solder wicking. However, this bulbous section also acts as a contact retention device for sidewards insertion into a multi-piece header. Because of the sidewards insertion limitation it would be virtually impossible to preload the contact arms.